1. Field of the Invention
The present invention relates to a Coriolis flow meter and method for determining a signal difference in cabling and first and second pickoff sensors.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as two independent pairs of wires. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
When the flow conduit or conduits of a Coriolis flow meter are empty, then the phase difference between the two pickoff signals is ideally zero. In contrast, during normal operation, the flow through the flow meter induces a phase shift between the two pick off signals due to the Coriolis effect. The phase shift is directly proportional to the material flow through the conduits. Therefore, by making an accurate measurement of the signal difference, the flow meter can accurately measure the mass flow rate.
Determining the signal difference between signals from the pickoff sensors is an important operation of the flow meter instrumentation. This signal determination must be accurately performed even though the cabling between the sensors and the instrumentation affects the measurement signals. All cabling includes inherent and distributed inductance, capacitance, and resistance characteristics. In addition, the pickoff sensors can have inherent characteristics that further affect the signal difference. Each pickoff signal must travel through the cabling and therefore the accuracy of the signal can be reduced before the signal reaches the measuring instrumentation of the flow meter.
Typical flow meter cabling can vary in length according to the environment and installation. A meter cabling can extend up to 1,000 feet. The distributed cable parameters, such as the inherent inductance, capacitance, and resistance, will introduce some signal difference onto a sinusoidal signal traveling through the cabling. As a result, at the end of the cable, two independent measurement signals traveling through the cable can experience a signal difference introduced by the cable if the signals do not experience the exact same cable parameters. Since the measuring instrumentation relates signal difference to mass flow, the cabling and sensor system adds an unwanted error term to the flow measurement.
In addition to mismatch between the two cable pairs, the distributed cabling and sensor system parameters will vary with temperature. This temperature variation can require a zeroing operation, such as when a flow meter is installed or when the ambient temperature changes by more that a certain amount. During a zeroing operation (i.e., under no-flow conditions), the instrumentation captures the signal difference generated by the system (including pickoff mismatch, cabling mismatch, instrumentation mismatch) and subtracts this offset from all subsequent phase measurements. However, a one-time zeroing does not guarantee proper operation, as the cabling/sensor system characteristics can and will change over time.
Prior art flow meters do not autonomously and continuously compensate for signal differences due to the inherent characteristics of cabling and pickoff sensors. Prior art flow meters do not perform compensation outside of the meter electronics.